For many years, defrosting food has been recognized as an advantageous application for microwave ovens. It has been appreciated that foods can be defrosted at a much faster rate in a microwave oven than if they are just permitted to sit on the kitchen counter. It is also well known that maximum or 100 percent power does not provide desirable defrosting or thawing. More specifically, thawed food is much more microwave absorptive than frozen food. Accordingly, once surface portions of the food thaw, they absorb most of the available microwave energy. If a high level of power such as, for example, 700 watts, is radiated at the food, the thawed portions will start to cook before the interior thaws. Because this is undesirable, most microwave ovens have at least one reduced power level that can be used for defrosting because it is considered to be a very important microwave application. For example, at a defrost setting, a typical microwave oven may provide 30 percent of its maximum power to limit the heating of the thawed portions.
One prior art approach to defrosting is to use maximum microwave power for 17 percent of the total time set and then reduce the power to 30 percent for the remaining 83 percent of the total time. Through the use of microprocessors or microcomputers, it has become a simple task to set a time period and have it divided into two percentage time periods and then to operate the microwave oven at predetermined power levels for those time periods. In this prior art approach, the set time period is to be taken by the operator from a defrost chart which is weight dependent.
In another prior art approach, the power is a stepped stair profile. For example, in a 15-minute defrost cycle, 80 percent power is applied for the first 3 minutes, then 60 percent power for the next 4 minutes, then 40 percent power for the next 4 minutes, and then 20 percent power for the last 4 minutes. In a 5-minute defrost cycle, 80 percent power is applied for the first 1.5 minutes, 60 percent power for approximately the next minute, 40 percent power for approximately the next minute, and then 20 percent power for the remaining time.
A third defrost approach uses either constant or stair-stepped reduced powers and provides periodic time intervals of no power for the heat to conduct from the thawed portions to the interior. One example is 50 percent power for the first minute, no power for the next minute, 27 percent power for the next 3 minutes, 9 percent power for the next 5.5 minutes and then 27 percent power for the next 4 minutes.
All of the above-described defrosting methods and profiles have significant drawbacks. For example, each either takes more time than is required to satisfactorily defrost the food or they begin to cook portions of the food while other portions remain frozen.